Polyurethanes are employed, for instance, in paints, in plastics, in elastomers, in foams, in adhesives and in many other materials. Polyurethanes are made by reacting isocyanates, mainly diisocyanates, with polyols, the isocyanate moieties reacting with hydroxy groups of the polyol to form urethane linkages. Isocyanates will also react with amines to form polyureas that find widespread applications in the same fields as polyurethanes. Isocyanates are therefore important compounds and the present invention provides a process for preparing isocyanates.
Urethanes can be regarded as esters of carbamic acids and one process for preparing isocyanates involves dealcoholysis of esters of carbamic acid, as shown in the following reaction: ##STR1##
Many attempts have been made to carry out this apparently simple process, but with only limited success. To achieve the required reaction it has been necessary to use stringent conditions, such as elevated temperature, usually above 300.degree. C., or toxic materials as catalyst, for instance powdered boron, powdered bismuth, boron carbide, boron nitride, germanium oxide, or both elevated temperature and toxic catalyst. Expensive solvents have also been required but even so yields of isocyanate have been low. The processes have also suffered from the tendency of the formed isocyanate to decompose at higher temperatures to give low yields.
In a paper in Tetrahedron Letters, 1985, Vol. 26, No. 11, pp 1411-1414, Robert K. Boeckman Jr. and Joan C. Potenza describe use of catechol boron halides (chloride and bromide) for cleaving certain ester, ether and carbamate protecting groups under mild conditions. Footnote 5 to the paper indicates that the product of cleavage of a carbamate is an amine.